I'm going to be that guy and point it out that the inflection of Mina* is wrong here, it should be ''Minun''.
If translated to english as is, it would be ''I big sister''. ''Minun'' is the genitive singular pronoun so if you say ''Minun isosisko'' it will be ''My big sister''.
*Mina should be written with ''Ä'', ''Minä''. Mina means nothing in Finnish. Though it is commonly mispoken/written by foreigners as Ä/Ö/Å vowel is not that common